Again, it's all well and good to say they could've done this instead of that, but they are the ones responsible for earning their cash, making their name and establishing their brand. Is it really that hard to understand that a sequel for a massively popular character might be a more attractive possibility than introducing a new, barely known character? Or to understand the fact that every new, unknown character getting their own movie brings a certain amount of risk, so doing too many of them at once is probably unwise?
None of that explains why they didn't put a Black Widow movie on the schedule as soon as the character became a breakout star in Iron Man 2 and The Avengers. Or why they took a risk on both Guardians of the Galaxy and Ant-Man before taking a risk on something that didn't have a white male lead.
Besides, look at how many characters Marvel has successfully introduced as supporting players in other movies -- Fury, Widow, Hawkeye, War Machine, Falcon, arguably Coulson, and now both Black Panther and Spider-Man. If they were worried about taking a risk on an unknown character, they could've seeded T'Challa or Carol or whoever in a movie several years ago in order to set up their solo film.
So the facts show that they are willing to take a risk on a new male lead character, but have been slow to feature even a popular female character that audiences were already clamoring to see in a solo movie. So if there's any fear of "risk" shaping their decisions, it seems to have more to do with the perceived risk of a non-white or non-male lead than with the risk of an untried premise.